Summary of Business and Significant Accounting Policies (Details)	1 Months Ended		12 Months Ended
	Dec. 31, 2020 USD ($) $ / shares shares	Nov. 30, 2020 shares	Apr. 30, 2021 USD ($) segment shares	Apr. 30, 2020 USD ($) shares	Apr. 30, 2019 USD ($)	May 01, 2019 USD ($)
Subsidiary, Sale of Stock [Line Items]
Reverse stock split ratio, common stock		0.1667
Deferred offering costs reclassified into stockholders' equity as a reduction of the IPO proceeds	$ 7,200,000
Restricted cash			$ 900,000	$ 500,000	$ 500,000
Allowance for doubtful accounts			800,000	800,000
Unbilled receivables			3,800,000	500,000
Long-lived assets, impairment			0	0
Goodwill, impairment			$ 0	0
Deferred costs, sales commissions, amortization period			5 years
Accrued liabilities, warranties			$ 0
Advertising expenses			35,300,000	29,200,000	5,200,000
401(k) matching contributions			$ 0	0	0
Number of operating segments | segment			1
Right-of-use assets			$ 5,094,000	8,409,000		$ 11,500,000
Total operating lease liabilities			$ 5,629,000	$ 9,180,000		$ 12,400,000
Operating Lease, Liability, Statement of Financial Position [Extensible List]						Other long-term liabilities
Deferred rent reduction						$ 900,000
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]			Other long-term liabilities	Other long-term liabilities
Minimum
Subsidiary, Sale of Stock [Line Items]
Property and equipment, useful life			3 years
Minimum | Trials
Subsidiary, Sale of Stock [Line Items]
Revenue, performance obligation			8 weeks
Maximum
Subsidiary, Sale of Stock [Line Items]
Property and equipment, useful life			5 years
Maximum | Trials
Subsidiary, Sale of Stock [Line Items]
Revenue, performance obligation			16 weeks
Sales and marketing | C3.ai DTI Grants Program
Subsidiary, Sale of Stock [Line Items]
Contribution expense			$ 0	$ 5,700,000	0
Research and development | C3.ai DTI Grants Program
Subsidiary, Sale of Stock [Line Items]
Contribution expense			$ 2,700,000	$ 5,700,000	$ 0
IPO
Subsidiary, Sale of Stock [Line Items]
Net proceeds received	694,600,000
Private Placement
Subsidiary, Sale of Stock [Line Items]
Net proceeds received	$ 150,000,000.0
Class A Common Stock
Subsidiary, Sale of Stock [Line Items]
Common stock, shares authorized (in shares) | shares		390,000,000	1,000,000,000	390,000,000
Class A Common Stock | IPO
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued (in shares) | shares	17,825,000
Sale of stock, shares issued, price per share (in dollars per share) | $ / shares	$ 42.00
Class A Common Stock | Over-Allotment Option
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued (in shares) | shares	2,325,000
Class A Common Stock | Private Placement
Subsidiary, Sale of Stock [Line Items]
Sale of stock, shares issued, price per share (in dollars per share) | $ / shares	$ 42.00
Class A Common Stock | Private Placement | Spring Creek Capital LLC
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued (in shares) | shares	2,380,952
Class A Common Stock | Private Placement | Microsoft Corporation
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued (in shares) | shares	1,190,476
Redeemable Convertible Class A-1 Common Stock
Subsidiary, Sale of Stock [Line Items]
Shares authorized (in shares) | shares		6,666,667	0	6,666,667
Conversion of stock, converted (in shares) | shares	6,666,665
Conversion ratio	1		1
Class B Common Stock
Subsidiary, Sale of Stock [Line Items]
Common stock, shares authorized (in shares) | shares		21,000,000	3,500,000	21,000,000
Preferred Stock
Subsidiary, Sale of Stock [Line Items]
Shares authorized (in shares) | shares		233,107,379	0	233,107,379
Redeemable Convertible Preferred Stock, Excluding Series A Preferred Stock
Subsidiary, Sale of Stock [Line Items]
Conversion of stock, converted (in shares) | shares	33,628,776
Conversion ratio	1
Series A*
Subsidiary, Sale of Stock [Line Items]
Shares authorized (in shares) | shares				21,000,000
Conversion of stock, converted (in shares) | shares	3,499,992
Conversion ratio	1